Wireless device detection system

ABSTRACT

A device for detecting wireless signals and locating the place of origin in a building is provided that includes a transmitter system using a cellular protocol; a plurality of antennae that receive wireless Radio Frequency (RF) signals, each antenna having a port; a receiving system connected to the antennae ports with a plurality of cables, converting the RF signal to voltage waveform; a processing system in communication with the receiving system, detecting the strength of the received signals and Time Difference of Arrival of the RF signals with respect to antennae and determining the location where the wireless signals are being emitted by using classification algorithms; and a human machine interface.

This application claims priority from and is a non-provisionalapplication of U.S. Provisional Application No. 61/792,155 filed on Mar.15, 2013, the complete disclosure of which is incorporated herein byreference.

This invention was made with government support under Contract Nos.2007-RG-CX-K179 and 2011-IJ-CX-K002 awarded by the National Institute ofJustice; and Contract Nos. N00014-04-M-0253 and N00014-06-C-0044 awardedby the Office of Naval Research. The government has certain rights inthe invention.

BACKGROUND OF THE INVENTION

This invention relates to a system for detecting and locating the placeof operation of wireless devices such as cellular phones, and inparticular, a system capable of detecting and locating the use ofcellular phones in an area such as a prison where radio frequency (RF)antennae have been installed at various locations.

It can be very important to be able to detect the use of and location ofwireless devices, such as cellular phones. One particular applicationwhere it can be very important to locate the use and operation of cellphones, and in particular, the use and operation of unauthorized cellphones, is in correction facilities. Numerous approaches have beenundertaken to prevent unauthorized use of cell phones in correctionfacilities, but this has become a continuing problem, which has beendifficult to address. Phones are smuggled in to inmates by guards orfamily members and activated using prepaid calling plans that are veryhard to trace. Such unauthorized use poses a severe concern as criminalsmay contact gang members or others to plan and coordinate illegalactivities including, but not limited to, buying drugs, elimination orintimidation of witnesses or competing gang members, and/or to planescape attempts, all while behind bars.

One prior approach for controlling unauthorized dissemination and use ofcell phones in correction facilities is to manage and carefully screenaccess. Such screening measures create additional costs and have beenminimally effective. Accordingly, it is an object of the presentinvention to provide a system and method wherein the use of a cell phonein a monitored area is detected, and based upon the signals detected,the system can calculate and pinpoint or approximate the location wherethe cell phone or wireless device is being operated.

SUMMARY OF THE INVENTION

In one embodiment of the invention, a device for detecting wirelesssignals and locating the place of origin in a building is provided thatincludes a transmitter system using a cellular protocol; a plurality ofantennae that receive wireless Radio Frequency (RF) signals, eachantenna having a port; a receiving system connected to the antennaeports with a plurality of cables, converting the RF signal to voltagewaveform; a processing system in communication with the receivingsystem, detecting the strength of the received signals and TimeDifference of Arrival of the RF signals with respect to antennae anddetermining the location where the wireless signals are being emitted byusing classification algorithms; and a human machine interface.

The transmitter system can be a modem(s) programmed through a computerto send out RF signals to provide training data sequences to theprocessing system. One of the antennae is denoted as the referenceantenna.

The receiving system may have four channels and samples the RF signalsand converts them to voltage using aliasing and band pass filtering. Thedevice may further comprise communication means between the receivingsystem and processing system. The communication means can be implementedusing Gigabit Ethernet.

The processing system may consist of a computer and signal processingsoftware, wherein the signal processing software may further includepreprocessing software. The preprocessing software filters voltagesignals and determines features of the voltage signals. The features ofthe voltage signals may include signal strength and Time Difference OfArrival to the antennae with respect to the reference antennae.

The signal processing software may further include classificationsoftware. Training data sequences can be used to provide statisticalinformation of features of voltage signals. A supervised classificationalgorithm may determine the location in which usage of the wirelessdevice occurred. A human machine interface may set an alarm and mayprovide an operator with visual information of the location in which thewireless device has been used.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of this invention andthe manner of obtaining them will become more apparent, and theinvention itself will be better understood by reference to the followingdescription of embodiments of the present invention taken in conjunctionwith the accompanying drawing, wherein:

FIG. 1 is a schematic diagram of the functional components of thewireless device detection system;

FIG. 2 is a map showing a layout of a building being monitored for cellphone use and location of the antenna to receive the transmittedsignals; and

FIG. 3 is a bar graph depicting success rate for detection of the use ofcell phones in various rooms in the building of FIG. 2.

Corresponding reference characters indicate corresponding partsthroughout the several views. Although the drawing representsembodiments of the present invention, the drawing is not necessarily toscale and certain features may be exaggerated in order to betterillustrate and explain the present invention. The exemplification setout herein illustrates embodiments of the invention, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

For the purpose of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings, which are described below. It will nevertheless beunderstood that no limitation of the scope of the invention is therebyintended. The invention includes any alterations and furthermodifications in the illustrated devices and described methods andfurther applications of the principles of the invention, which wouldnormally occur to one skilled in the art to which the invention relates.

Now referring to FIGS. 1 and 2, a system is depicted, generallyindicated as 10, for detecting and locating the source of wirelesssignals. System 10 includes a plurality of antennae 20, which are placedat strategic locations in a building, generally indicated as 12 (seeFIG. 2), wherein it is desired to detect the use and location ofwireless devices. System 10 also includes a processor card 22 that inone embodiment may be a Field-Programmable Gate Array (FPGA) board.System 10 also includes four front end receiver cards 24 that eachconnect to one antenna 20 through electrical cables 26, which may becoaxial cables. Analog to Digital Converters (ADC) 28 interface betweenfront end receiver cards 24 and the processor card 22. Filters 30 mayalso be placed between antenna 20 and receiver cards 24.

System 10 samples cellular bands at a high sampling rate simultaneouslyat different locations 20, and then uses signal processing methods todetect and locate the source of the signals. The present system has beenfound to be accurate within a few adjacent rooms or prison cells in agiven building. To process the signals, the processor board 22 runs areal time detection algorithm, described more fully below. The antennaeare placed inside the perimeter 14 of building 12 and are connected backto the board with coaxial cables 26. System 10 continuallymonitors/listens for wireless device usage/cell phone calls via downconverting receivers 24. In one embodiment, system 10 listens for callsin the 800 MHz cellular band, but the system is also suitable for othermajor bands used in the U.S., such as 1700 or 1900 MHz, or other bands.

When a cell phone signal is detected, the signals are sampled at eachantenna 20 and analyzed on a base computer 16. Specifically, an RFfingerprint is determined based on Time Difference Of Arrivals (TDOA)and received signal strengths. An RF fingerprint distilled from aspecific detection is compared to a previously recorded database offingerprints to locate the source of the signal/cell phone device.

The TDOA operates based on the time differences at which the cell phonesignal reaches the different antennae. In open environments with littlemultipath, the measured TDOA corresponds to a geometric time-of-flight.This is not true for indoor settings, wherein many path obstructions andreflectors can be present. System 10 can measure time differences forCode-Division Multiple Access (CDMA) and Global System for Mobile (GSM)signals to better than 5 nsec (which corresponds to a distance of 5feet); however, due to multipath, TDOAs cannot be used straightforwardlyto locate the cell phone. Instead, system 10 uses RF fingerprintingbased on the fact that RF signals emitted by a cell phone from a certainlocation, and received at another specific location are: 1) repeatable,2) vary little if the phone location samples one room/cell, and 3) varysignificantly from one room to another. RF fingerprinting is best suitedto a high multipath environment, but it requires a thorough calibrationat installation. It also can locate a cell phone to a specific prisoncell.

Ray tracing is one simulation method that may be used to extend thecurrent localization algorithm. Ray tracing algorithms, which can takefull advantage of detailed knowledge of the 3D construction of abuilding when such information is available, can provide betterperformance at the cost of higher computational complexity and technicalrisk.

Filters 30 may be a 100^(th) order Finite Impulse Response (FIR)band-pass filter. Computation of signal strength is straightforward asit is defined as the absolute mean of received voltages. However, to getthe TDOA between two received signals, the correlation peak of thesignals must be determined. To do this, the received signals arecorrelated, after having gone through the 100^(th) order band-passfilters 30. It is difficult to find an accurate peak of this correlationfunction because it is both signal and negative and yet not downconverted. Down conversion is not used because the channel, which thecell phone is transmitting on, is not known, and it is difficult innoisy conditions to determine this with enough accuracy to avoid phasewrapping. However, the channel frequency can be removed without knowingits exact value by low pass filtering the square of the correlatedsignal. The correlation must be squared in order to present a DCcomponent to low pass filter. A sharp 400^(th) order FIR low-pass filterwith a cutoff of half the baseband frequency (1.2288 MHz) is effectivein removing the channel frequency from the signal. A last step is tosmooth the correlation curve using a spline function. This is requiredbecause the received data is sampled at 62.5 MHz, or 16 nsec betweensamples, which does not provide enough resolution to find the timedifference bound to the necessary sub-nsec resolution. The peak index ofeach filtered and smoothed correlation is found, and these indexes areconverted to delays in nanoseconds using a linear function.

Before performing the analysis, it is best to remove outliers, althoughthe criteria for classifying a reception as an outlier are based onheuristics, to some extent. The outlier removal process takes place intwo steps. The first is to throw away any observation if any of the fourreceiving antennae gets the transmission outside the frequency band ofinterest (i.e., CDMA2000 channel number 490-530). This criterion isbased on the fact that the modem is programmed to use the CDMA2000channel 507. Once the outliers have been thrown away based on thechannel information, the mean (μ^(i) _(l)) and standard deviation (δ^(i)_(l)) of a particular feature “i” obtained from a particular location“l.” Then observations are thrown away where either of the seven featurevalues lies outside the (μ^(i) _(l)±3δ^(i) _(l)) region. The two-stepoutlier rejection process provides a refined data set of size 5099 froma set of 5880 observations. In other words, 13 percent of the collecteddata has been rejected as outliers.

Once the data have been gathered, free from outliers, classificationalgorithms can be used to achieve the classification goal. All but oneapproach solves the classification by maximizing the posteriorprobability. Both linear and non-linear classification methods are used.

A sample test of system 10 was set up in building 12 shown in FIG. 2. Acellular modem (multitech MTCBA-C1), using the CDMA 2000 protocol overthe Verizon network, was used as a transmitter as well as severalVerizon phones. As shown in FIG. 2, antennae 20 were located approximatethe corners of building 12. Note, as the same hardware is used to sampleall four channels, synchronization is not an issue after calibrating forsmall differences in the length of cables 26. Although building 12 usedfor the test is not a correctional institution environment, it issimilar in that: there is no line of sight between the transmitter andreceiving antennae, and the environment is not static (i.e., doors openand close and furniture and other items are moved around periodically).Building 12 is different from a correctional institution/prison;however, in that a prison has metal doors instead of wood, concretewalls instead of drywall/wood, and larger open spaces or hallways. Inspite of the differences, building 12 is still suitable and convenientto collect a large amount of data for making conclusions regarding thesuitability of the subject invention.

For the test application, the receiving system consisted of onereceiver/processor along with four antenna ports and four channels thatcan each sample at up to 400 Msps. The receiver is a down-convertingreceiver, and the processor was a Virtex-5 FPGA. Data were sent tocomputer 16, which ran Matlab via Gigabit Ethernet. The receiver boardcontained two major components, namely, a front-end down conversionconversion component for sampling signals in the RF band, low noiseamplification, and band pass filtering to approximately 90 MHz. Theother component is a Multi-Channel Digital Synthesizer and Processor(MCDSP) board to further down-convert the RF signal to approximately 20MHz, perform cell-phone energy detection algorithm, and send informationto computer 16 via the Ethernet.

The hardware down-converts received RF signals to an IF frequency. It iscapable of being configured in any of the cell phone bands in the U.S.by replacing RF filters (which are located in line with the receiveantennae 20), and by providing a suitable local oscillator frequency(provided by a flexible signal generator in the current design).

Extensive testing consisted of transmitting with a modem for theselected locations within each room (1-20) of building 12. The averagelocalization accuracy (percentage of identifying the correct room andthe source of the wireless signal) for varying numbers of detectingsignals and a number of positions in each room as calibration data is asshown in Table 1.

TABLE 1 Average location accuracy. Number of Number of sampled positionsin each office received signals 4 15 15 ~30 50% x x 1-3 x 60% 60% 10 x x80%Localization was deemed accurate if the software determined location ofthe transmitter within a neighborhood of three neighboring rooms (i.e.,a transmission from room 8 is accurate if deemed to be from room 7, 8 or9). It was found that sampling 15 positions in each room, and detectingthe signal ten times, enabled predicting the neighborhood of thetransmitter correctly in 80% of the time on average.

FIG. 3 is a bar graph showing the success rate in identifying usage fromthe room or a neighboring room for seven different rooms (room numbers6-13) in building 12. It is believed the lower success rate for room 13is due to the fact its neighbors are not included in the test. Multipletesting in different rooms has shown similar results. It should be notedthat classification accuracy is not affected significantly ifcalibration data taken from some time ago (two months) is used toclassify signals detected currently. Accordingly, the system has shownaccuracy of ±1 rooms/cells using recordings obtained over approximatelythree minutes of cell phone use.

While the invention has been taught with specific reference to theseembodiments, one skilled in the art will recognize that changes can bemade in form and detail without departing from the spirit and scope ofthe invention.

The invention claimed is:
 1. A device for detecting wireless signals andlocating the place of origin in a building, comprising: a transmittersystem using a cellular protocol; a plurality of antennae that receivewireless Radio Frequency (RF) signals, each antenna having a port andone antennae is denoted as a reference antenna; a receiving systemconnected to the antennae ports with a plurality of cables, convertingthe RF signal to voltage waveform; a processing system in communicationwith the receiving system, detecting the strength of the receivedsignals and Time Difference of Arrival of the RF signals with respect toantennae and determining the location where the wireless signals arebeing emitted by using classification algorithms, the processing systemincluding preprocessing software that filters voltage signals anddetermines features of the voltage signals including signal strength andTime Difference of Arrival to the antennae with respect to thereferenced antennae; and a human machine interface.
 2. The device ofclaim 1, wherein the transmitter system is a modem(s) programmed througha computer to send out RF signals to provide training data sequences tothe processing system.
 3. The device of claim 1, wherein the receivingsystem has four channels and samples the RF signals and converts them tovoltage using aliasing and band pass filtering.
 4. The device of claim1, further comprising communication means between the receiving systemand processing system.
 5. The device of claim 3, wherein thecommunication means is implemented using Gigabit Ethernet.
 6. The deviceof claim 3, wherein the processing system consists of a computer andsignal processing software.
 7. The device of claim 6, wherein the signalprocessing software further includes classification software.
 8. Thedevice of claim 7, in which training data sequences are used to providestatistical information of features of voltage signal.
 9. The device ofclaim 7, in which a supervised classification algorithm determines thelocation in which usage of the wireless device occurred.
 10. The deviceof claim 1, in which a human machine interface sets an alarm andprovides an operator with visual information of the location in whichthe wireless device has been used.
 11. The method for detecting wirelesssingles and locating the place of origin in the building as set forth inclaim 1, further including the step of providing indication meansimplemented using Gigabit Ethernet.
 12. A method for detecting wirelesssingles and locating the place of origin in a building, comprising thesteps of: providing a transmitter system that utilizes a cellularprotocol; providing a plurality of antennae that receive Radio Frequency(RF) signals, each antenna having a port; placing the antennae atdifferent locations spread throughout the building; providing areceiving system connected to the antennae ports; converting the RFsignal to voltage waveform; providing a processing system incommunication with the receiving system; detecting the strength of thereceived signals and Time Difference of Arrival of the RF signals withrespect to the antennae; determining the location where the wirelesssignals are being omitted by utilizing classification algorithms;providing signal processing software including preprocessing software,filtering voltage signals through the preprocessing software, anddetermining features of voltage signals including signal strength andTime Difference of Arrival to the antennae with respect to a referencedantennae; and providing a human machine interface.
 13. The method fordetecting wireless signals and locating the place of origin in thebuilding as set forth in claim 12, wherein the transmitter system is amodem(s) programmed through a computer, and further including the stepof sending out RF signals to provide training data sequences to theprocessing system.
 14. The method for detecting wireless singles andlocating the place of origin in the building as set forth in claim 12,including the step of denoting one of the antennae as a referenceantenna.
 15. The method for detecting wireless singles and locating theplace of origin in the building as set forth in claim 12, including thesteps of providing four channels on the receiving system, and samplingthe RF signals and converting the RF signals to voltage using aliasingand band pass filtering.